Fermi LAT detection of enhanced gamma-ray emission from the Crab Nebula region

ATel #4239; Roopesh Ojha (NASA/GSFC), Rolf Buehler (SLAC/KIPAC), Elizabeth Hays (NASA/GSFC) and Michael Dutka (Catholic Uni.) on behalf of the Fermi Large Area Telescope Collaboration
on 5 Jul 2012; 13:53 UT
Credential Certification: Roopesh Ojha (Roopesh.Ojha@gmail.com)

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The Large Area Telescope (LAT), one of the two instruments on the Fermi Gamma-ray Space Telescope, has observed a significant increase in the gamma-ray activity from a source positionally consistent with the Crab Nebula on July 3, 2012.

Preliminary LAT analysis indicates that the daily-averaged gamma-ray emission (E >100 MeV) from the direction of the Crab doubled from (2.4 +/- 0.5) x 10^-6 ph/cm2/sec (statistical errors only) on July 2nd to (5.5 +/- 0.7) x 10^-6 ph/cm2/sec on July 3rd, a factor of 2 greater than the average flux of (2.75 +/- 0.10) x 10^-6 ph/cm2/sec reported in the second Fermi LAT catalog (2FGL, Nolan et al. 2012, ApJS, 199, 31). All given fluxes are the sum of the pulsar and nebula emission. Preliminary analysis indicates flux variation on shorter time scales and for 3 of the 6-hour intervals on July 3, 2012 the average fluxes were at least (5.1 +/ 1.3) x 10^-6 ph/cm2/sec. A gamma-ray flux increase from the direction of the Crab Nebula was reported previously by the AGILE and LAT collaborations in September 2010 (ATel #2855 and ATel #2861) and again in April 2011 (ATel #3276, ATel #3282, ATel #3284, ATel #3286). This is the highest gamma-ray flux on daily scales since the extreme outburst in April 2011.

Fermi has interrupted all-sky scanning mode starting at 2012-07-04 23:24:43 UTC to observe the Crab Nebula and is expected to remain in this observing mode for up to 300 ks, depending on when the flare fades. This source is one of the "LAT Monitored Sources" and consequently a preliminary estimation of the daily gamma-ray flux observed by Fermi LAT is publicly available (link: http://fermi.gsfc.nasa.gov/ssc/data/access/lat/msl_lc/ ). We strongly encourage further multifrequency observations of that region though it should be noted that the source is 20 degrees from the Sun. For this source the Fermi LAT contact person is Rolf Buehler (buehler@stanford.edu).

The Fermi LAT is a pair conversion telescope designed to cover the energy band from 20 MeV to greater than 300 GeV. It is the product of an international collaboration between NASA and DOE in the U.S. and many scientific institutions across France, Italy, Japan and Sweden.